Correct posture of the body is important to avoid stiffness and injury as one goes about his daily affairs. Therefore, it has been proposed to use electronic devices to monitor posture and to provide feedback for posture correction. In many of such devices, at least one accelerometer is used to monitor the posture in real time. U.S. Pat. No. 5,749,838 (Kline) discloses such a posture monitoring device which is attached to the back of a user by a belt and which is aligned to the user's midsagittal plane. The midsagittal plane is defined as an imaginary vertical plane which cuts centrally through a user's body from the back to front. The device can be attached to the level of the L4-L5 vertebrae or the S1-S2 vertebrae. Any change in curvature of the backbone within the midsagittal plane can be detected by the accelerometer in the device. The user's average pelvic angle as captured when he was walking is taken as the reference for good posture. Subsequently, the user is considered as adopting a good standing posture if his pelvic tilt falls within +/−5° of this average pelvic angle when he is standing. If the posture is detected to be bad, the device vibrates to alert the user of his bad posture.
Kline's device is unable to monitor the gait of the user when he is walking and running. However, it is just as important to runners to monitor their posture and gait in order to avoid injury and to improve running efficiency. In particular, Kline's device cannot monitor the balance between the user's left and right sides; postural analysis is entirely along the curvature of the spine towards the front or back of the user. Therefore, in other such devices, multiple sensors are used to monitor gait. In one example, an inertial sensor or accelerometer placed in a belt tied around the waist of a user is vertically aligned to his navel, which is the body's lateral centre, to monitor the midsagittal plane. Another sensor unit is tied around the chest of the user, also aligned to his navel. The ankles and wrists are also tied with a sensor unit each. Together, these sensor units measure characteristics of the gait of the user, such as dynamic mobility, including cadence, stride velocity, stride length, trunk rotation, turning duration and arm swing velocity. In particular, the centrally placed sensor units are able to monitor any lateral skew of force the user's steps and to determine if his gait is laterally balanced.
However, all these devices require the user to wear a belt or harness by which at least one sensor unit is positioned in his midsagittal plane, either on his chest or near his navel. Monitoring of lateral balance in gait becomes inaccurate if the sensor unit is displaced into misalignment to the user's navel. Unfortunately, it is difficult to maintain alignment of a sensor unit to the navel if the user is performing an exercise; movements of the user tend to rotate the belt about him. Therefore, these devices are not rugged enough to monitor a user's posture and gait accurately during rigorous and prolonged exercises, such as long distance running.
Accordingly, it is desirable to provide a device which is able to monitor the gait of a runner, and which has a possibility of mitigated risk of inaccuracy due to displacement of the device during exercise.